Playermodels and their portraits have the ability to have two colors that can be changed. Most people are familiar with "dm_base" method, but here I will show a much more powerful method to give greater control for your color remaps.

5.13 Update:as of the recent Sven Co-op update version 5.13 there was an update to
allow any model entity or NPC that has remap textures to be controlled
via "topcolor&bottomcolor" keyvalues using ranges 0~255. This
allows for wide variants of models without needing additional skin
textures (adding to filesize) or multiple models as some people have
done just for reskins.

While this video tutorial was originally
written for just playermodels, the same methods apply for NPC/other
model textures only without needing a portrait.